1. Field of the Invention
This invention relates to an organic electroluminescence device (to be referred to as xe2x80x9corganic EL devicexe2x80x9d hereinafter) to be used for a display or the like as light-emitting device and also to a liquid crystal device having a high electric current carrying ability and an excellent carrier injection property and adapted to be used for such an organic EL device.
2. Related Background Art
Intensive research efforts are currently being paid for developing applications of organic EL devices that can be used as light-emitting devices showing a quick responsiveness and a high light-emitting efficiency. FIGS. 1 and 2 of the accompanying drawings schematically illustrate the basic configuration of the organic EL device (see xe2x80x9cMacromol. Symp.xe2x80x9d 125, 1-48 (1997)). Referring to FIGS. 1 and 2, there are shown an organic compound layers 10, a metal electrode 11, a light-emitting layer 12, a hole transport layer 13, a transparent electrode 14, a transparent substrate 15 and an electron transport layer 16. As shown in FIGS. 1 and 2, organic EL devices generally have an organic compound layers 10 having a multilayer structure and arranged between a transparent electrode 14 formed on a transparent electrode 15 and a metal electrode 11.
In the instance of FIG. 1, the multilayer organic compound layer 10 include a light-emitting layer 12 and a hole transport layer 13. The transparent electrode 14 is typically made of ITO that has a large work function so that the device may show an excellent hole injection characteristic for injecting holes from the transparent electrode 14 to the hole transport layer 13. The metal electrode 11 is typically made of Al, Mg or an alloy thereof, which have a small work function so that the device may show an excellent electron injection characteristic for injecting electrons into the organic compound layer 10. The electrodes typically have a thickness of 50 to 200 nm.
The light-emitting layer 12 is typically made of an aluminum-quinolinol complex derivative that shows an electron transport property and also a light-emitting property. The chemical structure of Alq3 is shown below as a typical example of such derivatives. The hole transport layer 13 is typically made of a phenyldiamine derivative having an electron providing ability such as xcex1-NPD as shown below. 
An organic EL device having a configuration as described above shows an rectifying property when a voltage is applied thereto and injects electrons from the metal electrode 11 into the light-emitting layer 12 and holes from the transparent electrode 14 when an electric field is applied thereto in such a way that the metal electrode 11 and the transparent electrode 14 operate respectively as a cathode and anode. The injected holes and electrons are recombined to produce excitons in the light-emitting layer 12 and emit light. At this time, the hole transport layer 13 takes a role of blocking electrons to improve the recombination efficiency along the interface of the light-emitting layer and the hole transport layer, which hence increases the light-emitting efficiency.
With the arrangement of FIG. 2, an electron transport layer 16 is provided between the metal electrode 11 and the light-emitting layer 12. With this arrangement, the light-emitting operation and the electron/hole transporting operation are separated to further improve the carrier blocking effect and the light-emitting efficiency. The electron transport layer 16 may typically be made of a oxadiazole derivative. The above described organic compound layer, 10 is typically made to have a two-layered or three-layered structure with a total film thickness of about 50 to 500 nm.
In any of the above illustrated organic EL devices, the degree of luminance of emitted light of the device depends on the performance of injecting electrons and holes from the respective electrodes of the device. When amorphous materials such as Alq3 and xcex1-NPD are used in a manner as described above, it is believed that the device may not necessarily show a satisfactory carrier injecting performance because of the problem of interfaces of the electrodes/organic-compound layer.
On the other hand, attempts have been made to utilize the structural regularity of liquid crystal as will be discussed hereinafter for the purpose of improving the carrier injection characteristic and the carrier transport characteristic of the device.
Liquid crystal materials having a high carrier transporting ability includes discotic liquid crystal compounds and smectic liquid crystal compounds that have a well-ordered structure. These liquid crystal materials normally show a degree of mobility that is as high as 10xe2x88x926 to 10xe2x88x923 cm2/Vsec. It is expected to realize a high productivity and an excellent performance on the part of organic electroluminescence devices by using such liquid crystal compounds. Applications of such compounds to solid electrolytes are also being studied.
Some of the characteristic aspects of the carrier transport effect that can be achieved by using liquid crystal materials include the following.
(1) A high carrier transporting ability can be achieved by the regular spatial structure obtained by the orientation of liquid crystal itself.
(2) A high electron injecting property can be achieved as a result of orientation of the xcfx80 electron conjugate planes of liquid crystal molecules toward the electrode interface.
Reports on attempts for doping a material having a carrier transporting ability with a compound having an electron receiving property or an electron providing property relative to the organic compound layer are also known. They include the following.
(1) Yamamoto et al., Applied Physics Ltter Vol. 72, No. 17, p. 2147 (1998)
(2) Kido et al., Applied Physics Letter Vol. 73, No. 20, p. 2866 (1998)
The above reference (1) reports that the authors have succeeded in raising the luminance of emitted light by forming a hole transport layer, using a hole transporting polymer material prepared by mixing a salt containing SbCl6xe2x80x94 with a polymer material by 20 mol %, producing holes in the hole transport layer and thereby raising the carrier density.
The above reference (2) reports that the electron injecting performance is improved by doping the electron transport layer with metal Li.
Reports on doping liquid crystal materials include the following.
(3) Boden et al, J. Am. Chem. Soc. Vol. 116, No. 23, p. 10808 (1994)
(4) J. Material Science: Materials in Electronics 5, p. 83 (1994)
The above reference (3) reports that an n-type semiconductor whose main carriers are electrons is formed by doping a discotic liquid crystal compound having a tricycloquinazoline skeleton with potassium by 6 mol %.
The above reference (4) reports that a p-type semiconductor whose main carriers are holes is formed by doping a discotic liquid crystal compound having a triphenylene skeleton with AlCl3.
However, when devices, which may not necessarily be light-emitting devices, are formed by using a liquid crystal composition prepared by doping a liquid crystal material with an inorganic compound in a manner as describe above, there arises a problem that not only carriers (holes or electrons) that operate as electrons but also ionized (cationized or anionized) dopants move in the liquid crystal material to cause an ionic electric current to flow in the device when an external electric field is applied thereto.
An ionic electric current is generated when the dopant itself moves. It is poorly reversible in terms of current characteristics and therefore not only the initial performance but also the durability of the device becomes problematic. Particularly, since liquid crystal materials have properties that are intermediary between crystal and liquid, the generation of an ionic electric current is more serious if compared with amorphous materials and polymer materials.
Additionally, if a discotic liquid crystal compound is used as a liquid crystal material, the temperature range in which the compound can be utilized effectively is limited because the liquid crystal temperature is high. Furthermore, while a discotic liquid crystal compound needs to be oriented uniformly in a direction that agrees with the main axes of liquid crystal molecules for the liquid crystal material to obtain a high degree of electric conductivity, it is relatively difficult to orient the liquid crystal material so as to make it have a predetermined structure if compared with other liquid crystal materials because the device is disk-shaped.
On the other hand, a smectic liquid crystal compound provides an advantage that the orientation process is relatively simple and easy, because only liquid crystal molecules are required to be arranged in parallel relative to the electrodes if the main axes of liquid crystal molecules are randomly arranged in terms of direction. It is also characterized in that it provides a high electric conductivity with such a rough orientation structure.
However, smectic liquid crystal compounds are accompanied by a problem that the device prepared by using such a liquid crystal compound generates an ionic electric current to obstruct the proper function of the device when the compound is doped because the dopant itself moves as ions within the device.
In view of the above identified problems, it is therefore an object of the present invention to realize a carrier transport layer by using a technique of doping a liquid crystal material and suppressing the phenomenon that the dopant itself becomes a source of ionic electric current so as to selectively improve the performance of carriers that may be electrons or holes.
Another object of the present invention is to provide a liquid crystal device that is improved in terms of carrier transport ability and carrier injection performance and also an organic EL device that has an improved light-emitting efficiency by using a liquid crystal layer of such a liquid crystal device for the carrier transport layer.
In a first aspect of the invention, the above objects are achieved by providing a liquid crystal device comprising at least one liquid crystal composition layer sandwiched between said pair of electrodes, said liquid crystal composition being formed by doping a smectic liquid crystal compound with a Lewis acid compound, wherein said smectic liquid crystal compound has a hexagonal order structure.
Preferably, in a liquid crystal device according to the invention, the smectic liquid crystal composition has a molecular structure containing a tertiary amine. Preferably, said liquid crystal composition layer is formed by an injection method that utilizes spin coating or the capillary phenomenon.
In a second aspect of the invention, there is provided an organic electroluminescence device comprising a plurality of organic compound layers arranged between a pair of electrodes, one of said organic compound layers being a carrier transport layer composed of a liquid crystal composition formed by doping a smectic liquid crystal compound with a Lewis acide type compound, another one of said organic compound layers being a light-emitting layer.
Preferably, in an organic electroluminescence device according to the invention, a protection layer composed of an organic compound having a carrier transporting property of the same type as that of the carrier transport layer and a carrier conductivity different from that of the carrier transport layer is formed between the carrier transport layer, and the light-emitting layer and one of the pair of electrodes is formed in the form a plurality of pieces arranged on a substrate and driven by way of thin film transistors.